Just in time and place: NOS/NO system assembly in neuromuscular junction formation

Recent advances in the molecular, biochemical, and anatomical aspects of po stsynaptic membrane components at the neuromuscular junction (NMJ) are brie fly reviewed focussing on assembly, architecture, and function of the multi -subunit dystrophin-protein complex (DPC) and its associated nitric oxide ( NO)-signaling complex. Elucidation of unique structural binding motifs of N O-synthases (NOS), and microscopical codistribution of neuronal NOS (nNOS), the major isoform of NOS expressed at the NMJ, with known synaptic protein s, i.e., family members of the DPC, nicotinic acetylcholine receptor (AChR) , NMDA-receptor, type-1 sodium and Shaker K+-channel proteins, and linker p roteins (e.g., PSD-95, 43K-rapsyn), suggests targeting and assembly of the NO-signaling pathway at postsynaptic membrane components. NO mediates agrin -induced AChR-aggregation and downstream signal transduction in C2 skeletal myotubes while administration of L-arginine, the limiting substrate for NO -biosynthesis, enhances aggregation of synapse-specific components such as utrophin. At the NMJ, NO appears to be a mediator of (1) early synaptic pro tein clustering, (2) synaptic receptor activity and transmitter release, or (3) downstream signaling for transcriptional control. Multidisciplinary da ta obtained from cellular and molecular studies and from immunolocalization investigations have led us to propose a working model for step-by-step bin ding of nNOS, e.g., to subunit domains of targeted and/or preexisting membr ane components. Formation of NOS-membrane complexes appears to be governed by agrin-signaling as well as by NO-signaling, supporting the idea that par allel signaling pathways may account for the spatiotemporally defined posts ynaptic assembly thereby linking the NOS/NO-signaling cascade to early memb rane aggregations and at the right places nearby preexisting targets (e.g., juxtaposition of NO source and target) in synapse formation. Microsc. Res. Tech. 55:171-180,2001. (C) 2001 Wiley-Liss, Inc.